1. Field of the Invention
The present invention relates to a fuel system, and in particular relates to a fuel cell system recycling water directly from a cathode unit to a fuel reaction unit.
2. Description of the Related Art
A fuel cell (FC) is a voltage generator that converts chemical power directly to electrical power. Compared with conventional voltage generators, an FC has many advantages, such as lower pollution, lower noise, higher energy density, higher energy conversion efficiency, and as such FCs are considered to be environmentally friendly. FCs can be applied to portable electronic devices, domestic generators, transportation, military equipment, space industry, and the like.
FC operation varies by type. Take a direct methanol fuel cell (DMFC) for example, a methanol aqueous solution oxidizes in an anode catalyst layer to generate hydrogen ions (H+), electrons (e−), and carbon dioxide (CO2), wherein H+ is delivered to a cathode through an electrolyte, and e− is delivered to a cathode though external circuit. Oxygen (O2) is provided in a cathode unit, such that H+ reacts with e− and O2 and forms water (H2O). Conventionally, DMFCs have issues of methanol crossover, i.e. methanol and water molecules in an anode unit cross through the electrolyte to the cathode unit, which affect the efficiency of the fuel cell system. Typically, concentration of the methanol fuel is lower than 10% to reduce crossover, but the power efficiency of the fuel cell system is also reduced. As a result, water at the cathode unit is recycled to the anode unit, and the concentration of methanol fuel is increased, thus increasing the power efficiency of the fuel cell system.
FIG. 1 is a block diagram of a conventional fuel cell system 100. Fuel cell module 111 comprises an anode 120 and a cathode 130. A storage unit 122 stores fuel for reacting with anode 120. The fuel is delivered to the anode unit 121 using pump 123, reacting with anode 120, and recycled to storage unit 122. A gas is delivered to cathode unit 131 using pump 132, cooled by cooler 133, and then exhausted. Water is collected in recycling area 134, and delivered to the storage unit 122 using pump 135. A fuel tank 141 storing high concentration fuel is coupled to storage unit 122, wherein fuel concentration in storage unit 122 can be adjusted by delivering the high concentration fuel to storage unit 122 using pump 142. Three liquid and one air pump, a discrete fuel storage unit, a cooler and a water recycling area are required. The cost and size of the fuel cell are compromised, and not suitable for portable fuel cell design.
U.S. Pat. No. 6,698,278 provides a fuel cell, as shown in FIG. 2. FIG. 2 depicts a fuel cell system 200. Fuel cell module 211 comprises an anode 220 and a cathode 230. A storage unit 222 stores fuel for reacting with anode 220. The fuel is delivered to the anode unit 221 using pump 223, reacting with anode 220, and recycled to storage unit 222. A gas is delivered to cathode unit 231 using pump 232, and then delivered to the storage unit 222. An air exit 224 exhausting gas is located at the storage unit 222. A fuel tank 241 storing high concentration fuel is coupled to storage unit 222, wherein fuel concentration in storage unit 222 can be adjusted by delivering the high concentration fuel to storage unit 222 using pump 242. Although some components are eliminated from fuel system of FIG. 1, there are too many components to achieve a more compact design. Pump 223 must run continuously, which also impacts the power efficiency of the FC system.